In a laser such as a semiconductor laser utilized in optical communications or a solid-state laser utilized in laser processing or the like, when light reflected by an optical surface or a work surface outside a laser resonator returns to the laser element, the laser oscillation is destabilized. The destabilized laser oscillation causes a signal noise in the optical communications, or may destroy the laser element in the laser for processing. In order to block reflected return light so that the light cannot return to the laser element, an optical isolator is used. Normally, an optical isolator is constituted of a Faraday rotator, a polarizer, an analyzer, and a permanent magnet.
Heretofore, a terbium gallium garnet crystal (hereinafter referred to as TGG) and a terbium aluminium garnet crystal (hereinafter referred to as TAG) have been used for a Faraday rotator to be incorporated in an optical isolator for a high-power laser.
However, the TGG and the TAG each have a small Faraday rotation coefficient per unit length. Hence, in order to obtain a polarization rotation angle of 45 degrees so as to function as an optical isolator, the TGG or the TAG needs to have a long optical path length, and a large crystal having a length as long as approximately 6 cm has to be used. Moreover, in order to obtain a high optical isolation, it is necessary to apply a uniform and large magnetic field to the crystal, and strong and large magnets have been used. For these reasons, such optical isolators have been large in size. Further, since the optical path length is long, the shape of beams from the laser is distorted in the crystal, which necessitates an optical system for correcting the distortion, in some cases. Furthermore, since the TGG is expensive, there is a demand for a small and inexpensive Faraday rotator.
Meanwhile, a bismuth-substituted rare-earth iron garnet crystal film (hereinafter referred to as RIG film), which is exclusively used in the field of optical communications, has a Faraday rotation coefficient per unit length significantly larger than those of the TGG and the TAG. The use of an RIG film can greatly reduce the size of an optical isolator. However, it is known that when the wavelength of light used is shortened from the 1.3 μm region or 1.55 μm region, which are exclusively employed in the field of optical communications, to around 1.1 μm, which is employed for a laser for processing, a large amount of light is absorbed by iron ions in the RIG film. This light absorption causes the temperature to rise, consequently causing the performance deterioration.
In this respect, Patent Documents 1 and 2 have proposed methods for solving the problem of temperature rise of an RIG film.
First, Patent Document 1 proposes a method in which an RIG film is grown on one surface of a non-magnetic garnet substrate, and then the RIG film is used as a Faraday rotator with the non-magnetic garnet substrate. According to this method, heat is dissipated through the non-magnetic garnet substrate, and thus the temperature rise of the RIG film can be prevented. Note that, in the Faraday rotator having such a structure, light is reflected at an interface between the RIG film and the non-magnetic garnet substrate. However, the reflected light can be handled in the same manner as for light returned from the outside of the optical isolator because the RIG film is disposed at the laser element (light source) side, so that when the reflected light passes through the RIG film, the polarization rotates by 90 degrees. Hence, the isolation performance does not decrease.
Moreover, Patent Document 2 proposes a method in which a non-magnetic garnet substrate used to grow an RIG film is removed by grinding and polishing processes, and then heat dissipation substrates made of a sapphire crystal or a rutile crystal are respectively bonded to both surfaces of the RIG film to thereby prevent temperature rise of the RIG film.